Amazon today introduced an update to the Kindle app for iPad, iPad mini, iPhone and iPod touch, bringing X-Ray to Kindle reading apps for the first time ever. X-Ray enables you to explore the "bones of the book" - with a single tap, you can see all passages across a book that mention ideas, fictional characters, historical figures, places or topics that interest you, as well as more detailed descriptions from Wikipedia and Shelfari, Amazon's community-powered encyclopedia for book lovers. Here's more info on the new Kindle update:

For example, A Dance with Dragons, book 5 of the series from George R.R. Martin, has over 400 characters. Sometimes you'll come across a character who hasn't been mentioned in the previous 2,000 pages. You are going to want X-Ray to read this book - just tap on the X-Ray button when you're reading and you'll be able to view all of the characters, ideas, places and topics, and you can browse where each word/phrase is used in the chapter, the page or the full book. Using X-Ray makes it easy to remind yourself which character is which and when you last came across that character in the book.

X-Ray is built on Amazon's expertise in language processing and machine learning, access to significant storage and computing resources with Amazon S3 and EC2, and a deep library of book and character information.